Cell mediated hyperacute rejection (CMHAR) of allogeneic mouse skin is an experimental model which differs from (probably serum-mediated) hyperacute rejection of transplanted viscera (in man or experimental animals) with respect to kinetics and immune pathways. It resembles it with respect to prominent involvement of the vasculature of the transplanted tissue, and the marked acceleration of the rejection process. The present study is aimed at elucidating events at the target site, and at dissecting the immune pathway(s) involved in bringing CMHAR about. It involves the identification of in vivo and in vitro analogs reflecting parallel pathways. Specific antisera (e.g. anti Ly.t sera) are being used to characterize the effector cell(s) participating in CMHAR, in comparison to other, not hyperacute forms of rejection.